In numerous applications of addition-crosslinked silicone rubbers it is of particular importance for the silicone rubber not to be permanently deformed under pressure. For example, the ability of a seal to function would be reduced or completely destroyed if it were to behave plastically under pressure, since increasing plastic deformation would at the same time reduce the stress resulting from the compression of the silicone rubber, and it is this stress which guarantees the sealing function. Consequently, the plastic component of the deformation behavior under pressure of a silicone rubber used for such applications should be as small as possible. This material property can be quantified by means of the compression set in accordance with DIN 53 517 which enables different materials to be compared with one another. The compression set is determined by determining the thickness of a cylindrical test specimen before and after a compressive strain of 25% is maintained for 22 hours at a temperature of 175.degree. C. If complete recovery occurs, i.e. the thickness of the test specimen is identical before and after the loading procedure, the compression set is 0%; if, on the other hand, the compressive strain of 25% applied during the test remains after unloading, the compression set is 100%.
Addition-crosslinked silicone rubbers typically have a compression set of up to 70%. To lower this, it is customary to subject silicone rubbers to heat treatment subsequent to the crosslinking reaction. The heat treatment is a thermal after-treatment of the silicone rubber which comprises, for example, storage for 4 hours at 200.degree. C. with admission of fresh air. Very low compression sets can be achieved in this way, but shrinkage of the silicone rubber has to be accepted.
Known methods of reducing the compression set make use of the partial poisoning of the Pt catalyst, so that the addition-crosslinking silicone rubber compositions can still be crosslinked and these crosslinked silicone rubbers have reduced compression sets without heat treatment. For example, in EP-A-388 201 the compression set is reduced by admixing the addition-crosslinking silicone compositions with benzotriazole. However, the partial poisoning of the Pt catalyst leads to a reduction in the crosslinking rate of the addition-crosslinking silicone rubber compositions. Furthermore, the mechanical properties after compression for significantly longer than 22 hours and/or at elevated temperature, in particular the compression set, are unsatisfactory.